I just tried adding a driver to a
sverchok node property, but it won't show up in the graph editor. Is it not possible to do that? Why not?
As a developer of Sverchok i'd like to answer in the negative.
No F-Curves or Drivers for a slider/property from a custom NodeTree will show up in the Graph Editor. Custom NodeTrees are things like Animation Nodes and Sverchok, ie: anything not from a Shader Node Tree / Compositor Node Tree.
I asked Blender developers in IRC about enabling this for custom node trees, but it appears not so trivial or not a priority.
I think what you could do is make a custom property for a normal Empty object, and add a Driver and reference the specific node tree in the driver code. Can't confirm this works, but it's worth trying.